Current research into the utilization of oxide superconductive materials is directed to the sintering process (see Jap. J. Appl. Phys., Vol. 26, No. 5, 1987, pages L624 through L626). According to this process, starting powders (RE, i.e., a rare earth element including Y and oxides or carbonates of Ba and Cu) are mixed at a predetermined composition of REBa.sub.2 Cu.sub.3 O.sub.7, the mixture is calcined to form a calcined powder having a structure of REBa.sub.2 Cu.sub.3 O.sub.7-y, and this calcined powder is then molded and sintered to obtain a superconductive material. As an application of this process, there can be mentioned research by Toshiba into the forming of a wire by packing the calcined powder into a metal cover or the like (Jap. J. Appl. Phys., Vol. 26, No. 5, 1987, pages L865 and L866). Moreover, a process has been attempted in which the calcined powder is molded into a plate and the plate is sintered to obtain a shielding material. These trials, however, have not been practically utilized because the critical current density of the sintered body is low.
The process in which the starting material is heat-melted at a high temperature and gradually cooled is not used for the preparation of a superconductive material, although used for the growth of a single crystal. In this case, in general, the starting powder containing a considerable excess of Cu or CuBa is grown in an alumina crucible. As a typical instance there can be mentioned research by NTT (Jap. J. Appl. Phys., Vol. 26, No. 5, 1987, pages L851 through L853).
In currently available superconductive materials such as sintered bodies, only current densities of several thousand A/cm.sup.2 are obtained at a temperature of 77 K. and an external magnetic field He of O T (Tesla), and thus they cannot be put to practical use. For a practical utilization, the current density must be increased to about 10.sup.4 A/cm.sup.2 at a temperature of 77 K. and an He of several T.
An object of the present invention is to improve the characteristics by using a melting process different from the conventional process for the production of a superconductive material, and to make it possible to practically utilize a superconductive material of the YBa.sub.2 Cu.sub.3 O.sub.7-y phase (hereinafter referred to as "123 phase").
The main problems to be solved to enable a practical utilization are as follows.
1) Improvement of the current density (Jc) in a zero magnetic field and a magnetic field.
2) Improvement of the shapeability into a wire, a coil, a plate and the like.
3) Improvement of the mechanical strength.
In a sintered body obtained according to the conventional sintering process, since the particle size is as fine as several microns to several hundred microns, many grain boundaries exist in the interior of the sintered body: These grain boundaries have a weak superconductivity, and a large superconductive current in particles is restricted and reduced at the grain boundaries. Accordingly, in the sintered body, the Jc is small, and is greatly reduced to scores of A/cm.sup.2 in a magnetic field. Furthermore, processing of the sintered body after the sintering and bonding of sintered bodies, is very difficult, and moreover, the sintered body is defective in that it is inherently brittle.